This invention concerns improvements in and relating to the compensation of transmission losses in a telephone system.
In the public exchange system it is essential to provide for compensation for transmission losses due to attenuation of the transmitted signal over long distance trunk connections, and the sophisticated methods now in use for encoding transmitted signals on digital transmission systems and using pilot frequencies as references for measurement to allow appropriate compensation for the attenuation on analogue transmission channels are such that over any path that is connected over the major trunk network a substantially zero transmission loss can be achieved. However owing to the complicated and expensive equipment required for the compensation of such transmission losses, it has not hitherto proved economic to equip local line plant with such means for automatic transmission loss compensation and thus in any telephone connection there may be a significant variation of the transmission levels delivered to the local telephone terminals which derives mostly from the unknown loss over the local telephone lines.
Hitherto it has not been possible to provide economically for compensation of this latter transmission loss in the local network and only to control it to fall within acceptable limits by mandatory specification so as to be sufficiently low in relation to the prescribed level of the transmitted signal to provide an acceptable received signal level with acceptable cross-talk between channels under the worst case telephone connnection possible in the network.
However in response to a growing need for more sophisticated forms of telephone service an increased variety of telpehone line terminal equipments and services are to be expected in future development of the telephone system.
In spite of the variance in transmission loss to be expected over telephone connections such new services and facilities might be expected to include, for example, the need to couple existing local telephone terminals to other similar telephone line terminals so as to join together in tandem more than one telephone connection established via the public or a private telephone exchange network. This joining together may be by means of a private telephone exchange or other means or ancillary apparatus having the means to link two or more telephone connections in tandem so as to establish such tandem connection. In the circumstances of such tandem connections a cumulative transmission loss may arise due, for example, to the addition under "worst case" conditions of the respective transmission losses in the tandem telephone connections thus joined each of which under "worst case" conditions will, in the interests of telephone network economy, have been individually specified to permit the worst possibility of as high a transmission loss as can reasonably be tolerated alone.
In the "worst case" the total attenuation which can be experienced on such tandem connections will be the summation of the "worst case" conditions and the total can be expected to be too great to allow an acceptable level of communications.
In order to provide compensation for the transmission loss which occurs in such circumstances, and which is not capable of being assessed by the automatic transmission compensation means in the trunk circuits of the telephone system, it has been proposed to provide a customer controlled gain circuit which allows for the customer himself to vary the transmission gain in order to provide acceptable communication. Such an arrangement is described, for example, in U.S. Pat. No. 4,017,695. Such known arrangements, however, essentially depend upon the customer's assessment of the amplitude of transmitted signals and requires compensation of such signals by a trial and error method, wherein the customer repeatedly keys-in amplitude correction commands to vary the transmission gain until an acceptable level is reached. The operation of such a system is therefore not only tedious for the user, but requires a sophisticated understanding of the nature of the gain compensation required and an assessment of the gain of received signals, which may in any case not be derived from a source which provides a signal of consistent amplitude. Such a system, moreover, does not allow of the control of gain by the customer at a plurality of separate locations within the transmission path, such as may be necessary to provide appropriate gain compensation without exceeding the permitted level of signal at the output of an individual gain compensation amplifier.